Fission Stories #158
The owner of the Columbia Generating Station in Washington reported to the NRC on October 24, 2013, about a design problem affecting containment.
(click to enlarge)
The Columbia Generating Station has a boiling water reactor with a Mark II containment design. This design has a primary containment (consisting of the drywell and wetwell regions shown in the figure) that is completely surrounded by a secondary containment (called reactor building in the figure). In event of an accident, valves and dampers automatically close in all pipes and ductwork passing through the primary containment’s walls to bottle up radioactivity released from a damaged reactor core.
The pressure inside the reactor building is maintained at a slightly lower pressure than the ambient pressure outside. This configuration results in clean air leaking into the reactor building rather than radioactively contaminated air leaking out from it. For while primary containment gets “bottled up” during an accident, some leakage from it into the reactor building is almost certain to occur.
The problem encountered at the Columbia Generating Station involved the inability to adequately maintain the pressure inside the reactor building below ambient pressure. A thunderstorm caused the ambient pressure to decrease and the control system for the reactor building’s ventilation system did not respond to automatically adjust the incoming and exhaust flows rates as necessary. In its report to the NRC, the owner noted “The potential for these momentary pressure excursions will continue with the currently designed control system.” The company did not commit to replacing the control system or modifying it to enable it to function properly during inclement weather.
Our Takeaway
The system isn’t supposed to work only on sunny, calm days. NRC’s regulations, specifically Criterion 2 in Appendix A to 10 CFR Part 50 states:
Structures, systems, and components important to safety shall be designed to withstand the effects of natural phenomena such as earthquakes, tornadoes, hurricanes, floods, tsunami, and seiches without loss of capability to perform their safety functions. The design bases for these structures, systems, and components shall reflect: (1) Appropriate consideration of the most severe of the natural phenomena that have been historically reported for the site and surrounding area, with sufficient margin for the limited accuracy, quantity, and period of time in which the historical data have been accumulated, (2) appropriate combinations of the effects of normal and accident conditions with the effects of the natural phenomena and (3) the importance of the safety functions to be performed.
In this case, a thunderstorm adversely affected containment.
To be fair, the system that encountered this problem at the Columbia Generating Station is the normal ventilation system for the reactor building. In event of an accident, the normal ventilation system is automatically turned off and another ventilation system (called the standby gas treatment system) automatically starts running.
But also to be fair, the standby gas treatment system is typically tested during sunny, calm days. Whether it would or would not be impaired by inclement weather like the normal ventilation system cannot be established from fair weather testing. And the owner was silent in its report to the NRC about robustness of the standby gas treatment system during thunderstorms, prolonged cloudiness, wind chills, and other natural phenomena that, in theory at least, the plant is designed to endure.
The NRC should investigate why the company is not fixing the sluggish response of the control system for the normal ventilation system and whether the standby gas treatment system is also unable to function properly during inclement weather.